1. Field of the Invention
The present invention relates to a process for the preparation of 4-hydroxy-2,4,6-trialkyl-cyclohexa-2,5-dien-1-one, and, more especially, relates to the preparation of 4-hydroxy-2,4,6-trimethyl-cyclohexa-2,5-dien-1-one by oxidation of 2,4,6-trimethyl-phenol.
4-Hydroxy-2,4,6-trimethyl-cyclohexa-2,5-dien-1-one, more commonly known as mesitylquinol, is a compound having the structural formula: ##STR1## and which is a useful intermediate in the preparation of trimethylhydroquinone, the latter being an important industrial chemical employed directly as an antioxidant or as a starting material for the synthesis of vitamin E.
2. Description of the Prior Art
It is already known to the art to prepare mesitylquinol by oxidation of 2,4,6-trimethyl-phenol, hereinafter referred to simply as mesitol. Thus, E. Bamberger, Ber., 36, 2,028-41 (1903), has proposed to oxidize mesitol with Caro's acid in the presence of magnesium carbonate. This process, however, is of scant industrial value because of both the high cost of the oxidizing agent used and the resultant low yields of mesitylquinol.
The French Patent Application No. 2,177,990 also discloses a process for the preparation of mesitylquinol by oxidation of mesitol, with molecular oxygen or an oxygen containing gas. Although the conditions for carrying out this process are defined quite broadly, it has been found that the achievement of good yields of mesitylquinol is linked to the use of very high pressures of air or of oxygen, namely, of at least 100 kg/cm.sup.2, and to carrying out the reaction in an aqueous solution of an alkali metal base (sodium hydroxide or potassium hydroxide). French Application No. 2,177,990 further indicates that the sodium hydroxide or potassium hydroxide can be replaced by nitrogen-containing bases such as guanidine; however, it has been found that even while utilizing an oxygen pressure of 100 kg/cm.sup.2, the selectivity in respect of mesitylquinol does not exceed 29.3%. In the absence of alkali metal hydroxides, resorting to pressures of air or oxygen of at least 100 kg/cm.sup.2 does not make it possible to achieve selectivities, in respect of mesitylquinol, greater than 1.1% in the absence of any catalyst or greater than 26.6% in the presence of a cobalt derivative as the catalyst. Finally, the need to resort to such high pressures of air or oxygen constitutes an obstacle to the industrial utilization of the process described in the noted French Patent Application No. 2,177,990.